Transparent cylindrical objects, such as fiber preforms, optical fibers, light pipes, light tubes, etc., are used in a variety of optical applications. In many instances, it is desirable to know the refractive index profile of such objects. For example, optical fibers are formed by heating a fiber preform and drawing the molten end into a thin glass thread. The refractive index profile of the preform defines the refractive index profile of the resulting optical fiber, which in turn determines the waveguiding properties of the optical fiber. It is thus important to be able to accurately measure the refractive index profile of the fiber preform.
One technique for measuring the refractive index profile of a transparent cylindrical object utilizes the deflection angle of rays of light emerging from a transversely illuminated object. Typically this involves passing a radiation beam (e.g., a laser beam) through the object in a direction transverse to the object central axis and measuring the resulting beam deflection angles as the laser beam is scanned across the object. The collection of beam deflection angles relative to the input location of the scanned laser beam is commonly known as the “deflection function.” Having obtained the deflection function, mathematical methods are then employed to reconstruct the refractive index profile from the measured data. These mathematical methods are generally based on paraxial ray theory, whereby the refractive index profile is determined by applying an inverse Abel transform to the deflection function.
However, the abovementioned technique is not able to provide an accurate measurement of the refractive index profile of a simple homogeneous rod. The reason for this shortcoming is that there is a refractive index discontinuity at the boundary, or edge, of the rod that results in a surface refraction component to the measurement. The publication by Werner J. Glantschnig, entitled “Index profile reconstruction of fiber preforms form data containing a surface refraction component,” Applied Optics, Vol. 29, No. 19, Jul. 1, 1990 (the “Glantschnig publication”), which publication is incorporated by reference herein, explains the reasons why a refractive index discontinuity is not accurately reconstructed from the deflection function data.
While the Glantschnig publication proposes a method for measuring the refractive index profile, it requires measuring the deflection angle precisely at the edge of the object, which is difficult to the point of impracticality.